Electrolyte for lithium battery, and lithium battery containing electrolyte

ABSTRACT

An electrolyte for a lithium battery contains ethylene sulfite, which includes chloroethanol in an amount of not more than 1000 ppm. A lithium battery includes an electrolyte, which contains chloroethanol in an amount of not more than 1000 ppm.

CROSS REFERENCE TO RELATED APPLICATION

This is a divisional application of a patent application Ser. No.10/227,242 filed on Aug. 26, 2002.

BACKGROUND OF THE INVENTION AND RELATED ART STATEMENT

The present invention relates to an electrolyte for a lithium batteryformed of refined ethylene sulfite, and lithium battery containing theelectrolyte.

Ethylene sulfite is used as a raw material in various fields of organicsynthesis, and a solvent or an additive for an electrolyte of a lithiumsecondary battery or the like.

Examples of conventional methods for producing the ethylene sulfite are:

-   i) a method of reacting ethylene glycol and thionyl chloride;-   ii) a method of reacting ethylene glycol and dimethyl sulfite;-   iii) a method of reacting ethylene oxide and sulfur dioxide;-   iv) a method of depolymerizing polyethylene sulfite.

Among these methods, the method of reacting ethylene glycol and thionylchloride has been considered advantageous industrially due to safety andlow costs.

D. S. Bleslow and H. Skolnic, Chem. Heterocycl. Compound., 1966, 21-1,1describes the method of reacting ethylene glycol and thionyl chloridefor producing ethylene sulfite without solvent and catalyst.

However, ethylene sulfite produced by the above conventional methodscontains a large amount of impurities.

The ethylene sulfite produced by the conventional methods is unsuitablefor a solvent and an additive for an electrolyte for a lithium secondarybattery, since the ethylene sulfite contains impurities so much that theelectrolyte does not have good storage stability. For example, theelectrolyte containing the ethylene sulfite produced by the conventionalmethods tends to cause an increase of acid content during preservation.When the electrolyte is used for a lithium secondary battery, thebattery is increased in its internal pressure due to the electrolytewhich sometimes damages a canister of the battery.

Thus, there has been required ethylene sulfite having high purity inorder to improve performance of the battery.

SUMMARY OF THE INVENTION

It is an object of the present invention to overcome aforementionedproblems and provide an electrolyte for a lithium battery formed ofrefined ethylene sulfite, and lithium battery containing theelectrolyte.

Ethylene sulfite is produced by reacting ethylene glycol with thionylchloride, and the ethylene sulfite contains chloroethanol in an amountof not more than 1000 ppm.

A method is for refining raw ethylene sulfite which is obtained byreacting ethylene glycol with thionyl chloride by a process including arectification process A, wherein the method further has at least onepurification process which is conducted before or after therectification process A, and which is selected from the group consistingof a washing process with basic water, a dehydration process byrefluxing, a distillation (rectification B) process, and an absorptionprocess.

An electrolyte for a lithium battery consists of ethylene sulfite,wherein the ethylene sulfite contains chloroethanol in an amount of notmore than 1000 ppm.

A lithium battery has an electrolyte, wherein the electrolyte containsethylene sulfite containing chloroethanol in an amount of not more than1000 ppm.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

A chlorine content in the ethylene sulfite is found to have an influenceto the storage stability of the electrolyte, and that chloroethanolcontained in the ethylene sulfite significantly influences the storagestability of the electrolyte and stability of the lithium secondarybattery containing the electrolyte.

The refined ethylene sulfite can be produced by producing the raw orcrude ethylene sulfite by reacting ethylene glycol and thionyl chloride,and rectifying the crude ethylene sulfite to produce refined ethylenesulfite.

<Production Method of the Raw Ethylene Sulfite>

Reaction of ethylene glycol and thionyl chloride to produce the raw orcrude ethylene sulfite can be conducted either with or without a solventand/or catalyst.

Examples of the solvent are: hydrocarbon halogenides such as methylenechloride; esters such as ethyl acetate; nitriles such as acetonitrile;ethers such as tetrahydrofuran and dimethoxyethane; and aromatichydrocarbons such as toluene, but the solvent is not limitative thereto.

The catalyst may be at least one basic compound, and examples thereofare inorganic bases such as sodium carbonate, potassium carbonate,sodium hydroxide, potassium hydroxide and calcium hydroxide, and organicbases such as N-methyl piperidine and N-ethyl morpholine.

Reaction temperature can be in a range of −20 to 100° C., preferably of0 to 80° C. Reaction time can be in a range of 0.5 to 10 hours,preferably of 0.5 to 5 hours.

When the reaction product contains the solvent(s), the raw ethylenesulfite can be produced by distilling the reaction product. The rawethylene sulfite also contains unreacted ethylene glycol, by-producedchloroethanol, and other impurities. The raw ethylene sulfite containssalt(s) of a basic compound and hydrogen chloride when the reactionprocess employs a catalyst of the basic compound. The salt can beremoved from the raw ethylene sulfite by filtration, washing or rinsing,extraction and so on.

<Refining the Raw Ethylene Sulfite>

Raw ethylene sulfite which is obtained by reacting ethylene glycol withthionyl chloride is refined by a process including a rectificationprocess A, and by at least one purification process selected from thegroup consisting of a washing process with basic water, a dehydrationprocess by refluxing, a distillation (rectification B) process, and anabsorption process which is conducted before or after the rectificationprocess A.

<Simple Distillation Process>

The simple distillation process can be conducted under either normalpressure or reduced pressure, and can be conducted at an internaltemperature not higher than 100° C.

<Rectifying Processes (Rectification A and Rectification B)>

The rectifying process (rectification A and rectification B) can beconducted with a rectifying column normally having theoretical plates of2 to 15 preferably 3 to 10 under either normal or reduced pressure,preferably at an internal temperature not higher than 100° C., and at areflux ratio of about 3 to 15.

The rectification A and the rectification B may be conducted undereither the same condition or different conditions.

<Washing Process with Basic Water>

The washing process can be conducted with water or a basic waterdissolved therein a basic inorganic salt such as sodium hydrogencarbonate, and is continued until the water phase becomes neutral. Inthis washing process, the amount of washing Water or the number ofapplication is not limitative.

<Dehydration Process by Total Reflux Distillation>

The process is conducted at an internal temperature not higher than 100°C. and at a pressure not higher than 20 Torr, so as to discharge waterout of the system in the form of non-condensed gas. The time ofprocessing can be normally in a range of 1 to 5 hours.

<Absorption Process>

The absorption process employs an absorbent which may be complex metaloxide such as molecular sieves, activated carbon, or metal oxide such asaluminum oxide (Al₂O₃) and magnesium oxide (MgO). The amount of theabsorbent can be 10% or less to the ethylene sulfite, but is notlimitative thereto. The process can be either a batch or a continuousprocess.

<Combination of Refining Process>

The raw ethylene sulfite is refined preferably by the simpledistillation, the first rectifying process and at least one refiningprocess, wherein the refining process can be conducted either before orafter the first rectifying process. Combinations of the first rectifyingprocess and the refining process(es) include the absorbing process andthe rectifying process; the washing process, the dehydration process bytotal reflux distillation and the rectifying process: and the washingprocess, the dehydration process by total reflux distillation, theabsorbing process and the rectifying process.

The above method provides the refined ethylene sulfite. The refinedethylene sulfite contains chlorine in an amount not more than 500 ppm,preferably not more than 200 ppm, and also contains chloroethanol in anamount not more than 1000 ppm, preferably not more than 400 ppm.

<Determination of Total Chlorine Content in Ethylene Sulfite>

A sample is diluted with an inert solvent for ethylene sulfite (forexample, toluene) and is burned in an oxyhydrogen flame combustor. Theresulted product is absorbed in a water solution of hydrogen peroxide.Chlorine ion content in the water solution is determined by ionchromatography, and the total chlorine content is calculated.

<Determination of Contents of Ethylene Sulfite, Chloroethanol andEthylene Glycol>

A sample is diluted by an inert solvent for ethylene sulfite (forexample, toluene) and is subjected to a gas chromatograph (column:dimethyl polysiloxane type, detector: Flame Ionizaion Detector (FID)).The contents of ethylene sulfite, chloroethanol and ethylene glycol areexpressed in percentage by area respectively.

The ethylene sulfite containing not more than 1000 ppm chloroethanolaccording to the embodiment is useful for a solvent and an additive ofthe electrolyte for the lithium battery, especially for the lithiumsecondary battery.

The electrolyte for a lithium battery according to another aspect of theinvention contains at least one lithium electrolyte, at least onesolvent, and ethylene sulfite containing not more than 1000 ppmchloroethanol. The ethylene sulfite containing not more than 1000 ppmchloroethanol can be prepared according to the above-described method.The lithium electrolyte may be a lithium compound such as lithiumborofluoride, lithium phosphate hexafluoride, lithium perchlorate, andlithium trifluoromethanesulfonate. The solvent may be a usual one for anelectrolyte such as a cyclic carbonate including ethylene carbonate andpropylene carbonate; a chane carbonate including dimethyl carbonate andmethyl ethyl carbonate; an ether group including tetrahydrofuran and1,2-diethoxyethane; and a lactone group such as γ-butyrolactone. Theethylene sulfite containing not more than 1000 ppm chloroethanol can beused for either the solvent or the additive. The ethylene sulfite havingthe reduced amount of chloroethanol has a good storage stability, andthe acid content therein can be kept low for a long time of storage, sothat it is useful for the electrolyte for the lithium battery especiallyfor the lithium secondary battery. On the other hand, the electrolyteconsisting of ethylene sulfite containing more than 1000 ppmchloroethanol has inferior storage stability.

<Determination of Acid Content in Electrolyte>

A precisely weighed sample is dissolved in cold pure water, and then thewater is titrated by alkali where the indicator is BTB (bromothymolblue) changing yellow to blue. The amount of the titrant is converted tothe acid content expressed as hydrofluoric acid content.

The ethylene sulfite refined through the above processes is normallyreduced in content of unreacted ethylene glycol to 2000 ppm or less, sothat it is also useful for the solvent and the additive for theelectrolyte of the battery, preferably the secondly battery.

EXAMPLES AND COMPARATIVE EXAMPLES

Without further elaboration, it is believed that one skilled in the art,using the preceding description, can utilize the present invention toits fullest extent. The following embodiments are, therefore, to beconstrued as merely illustrative, and not limitative in any waywhatsoever, of the remainder of the disclosure.

The present invention is further illustrated by the following Examplesand Comparative Examples. The determination is conducted as follows.

<Determination of Total Chlorine Content in Ethylene Sulfite>

A sample was diluted with toluene and was burned in an oxyhydrogen flamecombustor. The resulted product was absorbed in a water solution of 3%hydrogen peroxide. Chlorine ion content in the water solution wasdetermined by ion chromatography, and the total chlorine content wascalculated.

<Determination of Contents of Ethylene Sulfite, Chloroethanol andEthylene Glycol>

A sample was diluted by toluene and was subjected to a gas chromatograph(GC 14A manufactured by Shimadzu Corporation having a column of HR-1) insuch a manner that the temperature was held at 80° C. for 5 minutes andthen raised to 220° C. at a rate of 8° C./min, and then held for 10minutes, and an injector and a detector (FID) were kept at 250° C. Thecontents of ethylene sulfite, chloroethanol and ethylene glycol wereexpressed in percentage by area respectively, where the limits ofdetection of chloroethanol and ethylene glycol were both 25 ppm.

<Determination of Acid Content in Electrolyte>

A precisely weighed 10 g sample was dissolved in 100 cm³ pure water of5° C. or lower, and then the water was titrated by alkali where theindicator was BTB (bromothymol blue) changing yellow to blue. The amountof the titrant was converted to the acid content expressed ashydrofluoric acid content.

<Production (I) of Raw Ethylene Sulfite>

1 kg of ethylene glycol (available from Mitsubishi Chemical Corporation)was held in a flask having a capacity of 2 liters. The flask wasevacuated to reduce the internal pressure to 30 Torr at a roomtemperature, and then nitrogen gas was introduced into the flask tosubstitute the atmosphere. After that, a total amount of 2.1 kg ofthionyl chloride (available from Kishida Chemicals Ltd., purity: 95%)was dropped into the flask for 6 hours, while agitating the contents inthe flask, through which the internal temperature rose up to 45° C.After finished dropping the thionyl chloride, the contents were aged atan internal temperature of 68° C. for 70 minutes, and then the flask wasevacuated to 35 Torr and kept at an internal temperature of 63° C. for25 minutes. The resulted reaction product was thin-brown liquidcontaining ethylene sulfite of 97.04%, chloroethanol of 0.47% andethylene glycol of 1.30%.

The product was simple-distilled at a pressure of 20 Torr with a heatmedium having a constant temperature of 85° C. The initial distillatewhich was 5% of the total distillate was cut off and removed therefrom.The product distillate thus distilled at a yield of 86% was the rawethylene sulfite which contained ethylene sulfite of 97.7%,chloroethanol of 0.28%, ethylene glycol of 1.6% and total chlorine of5300 ppm.

It should be noted that the raw ethylene sulfite thus obtained wasemployed as a starting raw ethylene sulfite in the later-describedExamples 1 to 4 respectively as well as the below-described rectifyingprocess, so that the raw ethylene sulfite is sometimes referred to asparent raw ethylene sulfite hereinafter.

The parent raw ethylene sulfite was rectified with a rectifying columnhaving ten theoretical plates and a heat medium having a temperature of91° C., wherein the reflux ratio was kept at 10 until 5% of thedistillate was distilled out initially, and after that, the ratio waskept at 5.

The refined ethylene sulfite distilled at a yield of 71.8%, had a purityof 99.30%, and contained chloroethanol of 0.14%, ethylene glycol of0.38% and total chlorine of 1600 ppm.

Example 1

The above parent raw ethylene sulfite was subjected to an absorbingtreatment with an activated carbon (Sekado BW-50, produced by ShinagawaChemicals Ltd.) as an absorbent, wherein the activated carbon was addedin the raw ethylene sulfite in an amount of 10% and then the ethylenesulfite was agitated for 5 hours. The ethylene sulfite was furtherrectified in the same manner as the above Production (I).

The ethylene sulfite was rectified at a yield of 68%, and had a purityof 99.5%, and contained chloroethanol of 0.09%, ethylene glycol of 0.18%and total chlorine of 480 ppm.

Example 2

The parent raw ethylene sulfite was added with a sodium hydrogencarbonate saturated solution in an amount of 30% by weight, was agitatedfor one hour and then left at rest for 30 minutes, and after that, thewater phase was removed. After the resultant was added with water in anamount of 20% by weight, it was agitated for one hour. After it was leftat rest for 30 minutes, the water phase was removed. The ethylenesulfite thus washed two times was dehydrated by dehydration process bytotal reflux distillation at an internal temperature of 70° C. and at apressure of 20 Torr for 5 hours, while discharging water out of thesystem. The dehydrated ethylene sulfite was further rectified in thesame manner as the Production (I).

The ethylene sulfite was rectified at a yield of 60.1%, and containedchloroethanol of 0.09%, ethylene glycol of 0.05% and total chlorine of110 ppm.

Example 3

The refined ethylene sulfite obtained in Example 2 was further treatedby the absorbing treatment with a molecular sieve of 5A as an absorbent,wherein the absorbent was added into the refined ethylene sulfite in anamount of 3%, and then the ethylene sulfite was agitated for 5 hours.After that, the treated ethylene sulfite was rectified in the samemanner as the Production (I).

The ethylene sulfite was rectified at a yield of 54%, and containedchloroethanol of 0.01%, ethylene glycol of 0.01% or less and totalchlorine of 40 ppm or less.

Example 4

440 kg of ethylene glycol (available from Mitsubishi ChemicalCorporation) was held in a reaction vessel having a capacity of 1 m³.The vessel was evacuated to reduce the internal pressure to 30 Torr at aroom temperature, and then nitrogen gas was introduced into the vesselto substitute the atmosphere. After that, a total of 900 kg of thionylchloride (available from Kishida Chemicals Ltd., purity: 95%) wasdropped into the vessel for 12 hours, while agitating the contents inthe vessel, through which the internal temperature rose up to 35° C.Having finished dropping the thionyl chloride, the contents were aged atan internal temperature of 68° C. for 70 minutes, and then the vesselwas evacuated to 95 Torr and kept at an internal temperature of 69° C.for 3 hours.

The reaction product was simple-distilled at a pressure of 20 Torr witha heat medium having a constant temperature of 85° C. The initialdistillate which was 5% of the total distillate was cut off and removedto obtain the raw ethylene sulfite. The raw ethylene sulfite was addedwith a sodium hydrogen carbonate saturated solution in an amount of 30%by weight, and was agitated for one hour. After it was left at rest for30 minutes, the water phase was removed. After the resultant was addedwith water in an amount of 20% by weight, it was agitated for one hour.After it was left at rest for 30 minutes, the water phase was removed.The ethylene sulfite thus washed two times was dehydrated by dehydrationprocess by total reflux distillation at an internal temperature of 70°C. and at a pressure of 20 Torr for 5 hours, while discharging water outof the system. The dehydrated ethylene sulfite was rectified with arectifying column having ten theoretical plates and a heat medium havinga temperature of 90° C., wherein the reflux ratio was kept at 10 until5% of the distillate was distilled out initially and after that, theratio was kept at 5. The refined ethylene sulfite was further treated bythe absorbing treatment with a molecular sieve of 5A as an absorbent,wherein the absorbent was added into the refined ethylene sulfite in anamount of 3%, and then the ethylene sulfite was agitated for 5 hours.After that, the treated ethylene sulfite was rectified with therectifying column having ten theoretical plates and a heat medium havinga temperature of 90° C., wherein the reflux ratio was kept at 10 until5% of the distillate was distilled out initially, and after that, theratio was kept at 5.

The ethylene sulfite was rectified at a yield of 62%, and containedchloroethanol in an amount below the limit of detection, ethylene glycolin an amount below the limit of detection, and total chlorine of 40 ppmor less.

<Stability of Electrolyte Added with the Above Ethylene Sulfite>

An electrolyte was prepared as follows: Ethylene carbonate of 286 g andethylmethyl carbonate of 514 g were mixed. Having been added with amolecular sieve 4A of 5.1 g as an absorbent, the mixture was dehydratedfor 5 hours. The mixture was then filtrated by filter of 1 μm. Thefiltered mixture was added with commercially available LiPF₆ of 114 glittle by little, and then stirred for 30 minutes. After that, themixture was filtrated by a filter of 1 μm again so as to prepare theelectrolyte. The electrolyte contained acid of 9.5 ppm and water of 5.1ppm.

Five samples i), ii), iii), iv) and v) consisting of the electrolytewere prepared, and each of the samples ii) to iv) was added withethylene sulfite in an amount of 2 wt % respectively.

To the first sample i), ethylene sulfite was not added.

To the second sample ii), ethylene sulfite which was prepared in Example3 containing chloroethanol of 0.01% was added.

To the third sample iii), ethylene sulfite which was prepared in Example3 and further the chloroethanol were added till its concentration inethylene sulfite became 0.21%.

To the fourth sample iv), ethylene sulfite which was prepared in Example3 and further added with the chloroethanol till its concentration inethylene sulfite became 0.35% was added.

To the fifth sample v), ethylene sulfite which was prepared in Example 4containing chloroethanol in an amount less than 25 ppm which was belowthe limit of detection was added.

Each of the samples i) to v) of the electrolyte was stocked in astainless steel (SUS No. 304: American Iron and Steal Institute)container and held at a temperature of 25° C. in an atmosphere ofnitrogen. Acid contents of the samples were determined on the first dayand after ten days. The results are shown in Table 1. TABLE 1Concentration of Acid content No. of chloroethanol in Initial acidafter10 days electrolyte ethylene sulfite content (ppm) (ppm) i) — 9.513.5 (No ethylene sulfite was added) ii) 0.01% 9.4 19.2 iii) 0.21% 13.833.2 iv) 0.35% 13.9 38.5 v) below 25 ppm 9.6 13.6<Stability of Batteries Containing the Electrolyte>

Batteries i) to v) are produced with the above electrolytes i) to v) anda battery casing having a size of 5 cm×9 cm×6 mm made of a laminate filmhaving an aluminum sheet and resin layers on both sides thereof. Thebatteries are kept at 25° C. for one month, and then observed theappearances thereof. The results will become as shown in Table 2. Itshould be noted that the serial number of the battery corresponds tothat of the electrolyte. TABLE 2 No. of battery Apperance i) Very slightexpansion is observed. ii) Very slight expansion is observed. iii)Expansion is observed. iv) Expansion is observed. v) Very slightexpansion is observed.

The refined ethylene sulfite of the invention includes a very smallamount of impurities, so that it will exhibit excellent storagestability when it is added to an electrolyte for a battery.

The foregoing is considered illustrative only of the principles of theinvention. Further, since numerous modifications and changes willreadily occur to those skilled in the art, it is not desired to limitthe invention to the exact construction and operation shown anddescribed. Accordingly, all suitable modifications and equivalents maybe resorted to that fall within the scope of the appended claims.

The disclosure of Japanese Patent Application No. 2001-259262 filed onAug. 29, 2001 is incorporated herein as a reference.

1. An electrolyte for a lithium battery consisting of ethylene sulfite,said ethylene sulfite containing chloroethanol in an amount of not morethan 1000 ppm.
 2. A lithium battery comprising an electrolyte, saidelectrolyte containing chloroethanol in an amount of not more than 1000ppm.
 3. An electrolyte according to claim 1, wherein said ethylenesulfite is obtained by a method of reacting ethylene glycol with thionylchloride by a process including a rectification A, and providing atleast one process which is another purification selected from the groupconsisting of washing with basic water, dehydration by refluxing,distillation (rectification B), and absorption before or after therectification A.
 4. An electrolyte according to claim 3, wherein saidmethod further comprises a simple distillation process of said rawethylene sulfite or rectified ethylene sulfite which is conducted beforesaid refining process.
 5. An electrolyte according to claim 3, whereinsaid absorbing process employs at least one absorbent selected from thegroup consisting of complex metal oxide, activated carbon and metaloxide.